1. Field of the Invention
This invention relates to an ignition system for the internal combustion engine, or more in particular to a spark duration control system.
In a spark ignition engine, ignition is difficult under the conditions of low speed and small load because of a considerable amount of residual combusted gas. In view of the recent demand of the purification of the exhaust gas and the use of a leaner mixed gas, on the other hand, the importance of improving the ignition performance at the low speed under small load is increasing.
One of the existing methods to cope with this problem consists in lengthening the duration of the spark. If the spark life is lengthened to about 3 msec, for instance, the ignition ability is improved greatly under the above-mentioned conditions as known well.
Such a long spark duration, however, is not only useless under the condition of high speed or large load where the ignition characteristic is excellent but also greatly expedites the wear of the electrode of the ignition plug.
Our researches show that the wear of the electrode of the ignition plug is largely attributable to the discharge after the top dead center (TDC) for compressive motion of the piston of the internal combustion engine, while the discharge before the top dead center for compressive motion does not substantially contribute to the wear of the electrode. This is considered due to the fact that the discharge under a high pressure, high temperature and a flame accelerates the electrode wear.
Our researches also revealed that the discharge after the compression top dead center did not contribute to the ignition at all.
Thus an ignition system is required by which the spark is not kept on after the top dead center for compression and the spark is kept on as long as possible under the conditions of low speed and low load.
2. Description of the Prior Art
A system similar to the present invention is disclosed conventionally by the U.S. Pat. No. 3,896,776. This system is such that the primary coil is cut off only during the period from ignition to the top dead center while the primary coil is supplied with power for all the other periods. According to the characteristic diagram of this system, for the revolutional speed of 6000 rpm, the engine is ignited for every half a revolution thereof. In this case, the primary coil is turned on for about 4.3 ms and turned off for 0.7 ms, resulting in the off-time to on-time ratio of 16%. The situation becomes worse when the engine revolutional speed is reduced to 600 rpm, where the on-time of 48.3 ms and the off-time of 1.7 ms are involved, thus leading to the off-time to on-time ratio of 3.5%. This excessive length of on-time will heat and finally burn the ignition coil. This conventional system, therefore, cannot be actually mounted for use on the automobile.